Coated paper and method of making same



Patented Sept. 5, 1950 2,520,900 COATED PAPER ANSIQIIVIIETHOD OF MAKING Frederick H. Frost, Portland, Maine, assignor to S. D. Warren Company, Boston, Mass., a corporation of Massachusetts N Drawing. Application July 29, 1946, Serial No. 686,944

Claims. 1

This invention relates to twice-coated papers wherein an aqueous mineral coating layer overlies a previously applied and dried mineral coating base layer. While the invention is especially concerned with those twice-coated papers of which the mineral component of the top coating layer comprises finely divided metal (e. g., aluminum) it is not, in its broadest aspects, limited to metal as the mineral component of the top layer.

In the art of coating paper it is fairly common practice to apply to a paper base a coating composition comprising pigment material; e. g., clay, and an adhesive or binding material, e. g., casein; to dry said coating; and subsequently, either without or with an intermediate calendering step, to apply another coating on top of the first coating, said second coating being identical with or different from the coating first applied. Such procedure finds its most obvious use in situations in which the top coating is relatively expensive and in which the use of a base-coat makes possible a reduction in the thickness of the topcoat required. It is well known that when identical coatings are used, the application of two successive coats gives a product of better and more uniform quality than does the same weight of coating applied in a single operation.

However, when dissimilar coatings are being employed it frequently happens that the top coating strikes into the base-coat to an undesirable extent; also that th absorption of the top coating into the base-coat is non-uniform over the web, giving rise to streaks, mottle. unevenness, and other highly objectionable visual characteristics in the surface of the final product. Thus, in the case of an aluminum-pigmented, water-borne coatin overlying a conventional base-coat of mineral paper coating composition, the product is wont to have a pronounced unevenness of luster (that is to say, it exhibits dull streaks interspersed with lustrous areas all over the surface).

Moreover, because in the absorption phenomenon it is the fiuid component of the top coating composition (viz., the aqueous solution or dispersion of adhesive) which is absorbed, the above referred to product suffers the further disadvantage that the pigmentary component of the top layer is insufficiently and non-uniformly bound to the base-coated web, with the result that in places where absorption has been greater the top layer is weak to dry rubbing and is not satisfactorily resistant to rubbing when wet.

It has now been found that the tendency for the uneven (and undue) absorption of the topcoating into the under-coat in twice-coated papers can be controlled by incorporating a small but effective amount of a finely dispersed waxy material of low acid value into the aqueous mineral coating composition to be used for the base coating. By small but effective amount is here meant a. quantity suflicient to insure uniform hold-up of the subsequently applied aqueous top-coating composition while being insufllcient to make the base coat dimcultly wettable by said top-coating composition.

The principles of the invention are typically exemplified in the following comparative experiments: There was provided a conventional kraft paper body stock, suitably sized and smoothed, and having a basic weight 25 38500 of about 55 pounds. To one side of this web was applied 13 pounds dry weight of an aqueous mixture containing clay parts, alkali dissolved casein 11 parts, 5 parts of parafiln wax of 138 M. P. finely dispersed in the presence of sodium stearate and casein, and a little formaldehyde to water-proof the casein when subsequently dried. The so-coated web was dried. To the coated side was applied about 2.5 lb. dry weight of an aqueous mixture containing 100 parts of aluminum flake of litho lining grade, 20 parts of alkali-dissolved casein, and a little dimethylol urea to waterproof the casein. The coatedweb was then dried in a conventional manner. The coated sheet so produced was of uniform luster all over its surface. The top-coating was bound securely when tested by rubbing, and it was satisfactorily re sistant to rubber after being wet with water.

r When suitably supercalendered, it had an emissivity of not over 0.15.

In the comparison experiment, another sheet was prepared in exactly the same way except that the wax dispersion was omitted from the base coat. This sheet, uncalendered, was of uneven luster showing dull streaks interspersed with lustrous areas all over its surface. Moreover, in places it was weak to dry-rub. It was not satisfactorily resistant to rubbing when wet. The advantage resulting from use of wax in the base coat is obvious.

Paramn wax, of the above example, may be replaced by other mineral waxes such as: ceresin, montan, ozokerite, micro-crystalline petroleum wax, or other natural or synthetic waxes.

Spermaceti, for example, seems to be slightly more effective than paramn but, of course, is more expensive to use. A waxlike hydrogenated castor oil is also satisfactory. In fact, it appears that any waxy substance of negligible acid value, is usable in cases where the pH value of both coatings is not over 8. For very alkaline coatings the use or completely unsaponifiable waxes such as parafiln is indicated. If the pH of either coating is 8 or above, as will be true in the case of most conventional aqueous coating compositions, substances of high acid value (such as rosin or stearic acid) do not give results at all comparable with those given by paraifin or other neutral waxes, and their use is excluded. According y. the preferred waxy substances are those which are unsaponifiable or are saponifiable only with difiiculty.

In the example cited, in which an aluminum coating was applied over a clay coat, the wide difference in the natures of the two coats makes the eflect of the wax addition in the base-coat very easily discernible. A comparable benefit,

though one not so apparent to casual observation, results from use of wax in an under-coat used to support a top coat of similar or generally identical characteristics. In a sheet for decorative use, e. g., a box cover, uniformity in surface appearance is obviously to be desired. Likewise, in a sheet intended for printing purposes, uniformity of printing surface is equally highly desirable. The present invention is useful in both those classes of paper. thereon two coatings of quite similar nature is probably the best paper for letter-press printing made in this country today. Use of wax in the base-coat thereof, in accordance with this invention, is beneficial in maintaining uniformity in the printing surface.

; The proportion of wax to other base-coat solids may be varied between wide limits depending upon the particular results required, the particular wax used, the nature of each of the two coatings, and possibly other variables. In general, it may be said that 1% of wax, based on the weight of the total solids, in the base coat gives a noticeable improvement; that from 4% to 10% usually gives as much benefit as can be expected; and that in the range from 10% to it may be found that the top-coating is somewhat repelled by the basecoat unless the top-coating contains an eflicient wetting agent. About 5% of wax has been found efiective and safe to use in all cases tried; there seems to be no reason to use over 15%.

In the case of the aluminum coated paper previously described, casein was used as the adhesive. 0f course, the casein could be replaced by other 1 known water-dispersible adhesives and the prod- 1 f uct would still be benefited by the use of wax in y'the base-coat. For instance, the adhesive in one or both coatings may be one or more of the following: casein, starch, animal glue; soy protein, methyl cellulose, a synthetic elastomer (such as butadiene-styrene copolymer latex), gum arabic, alginic acid, zein, or the like.

Likewise, the pigment used in either coat may be varied as desired. Clay, calcium carbonate, barium sulfate, titanium dioxide, metal powder, talc, zinc stearate and the like are all suitable for use in one or both coatings. The top-coating may be entirely without pigment if desired. The basecoat, however, shall contain pigment enough to make up at least half the dry weight of said basecoat.

An example of a printing paper embodyingthe invention is the following: A paper basei was prepared for a furnish containing 2 parts of bleached soda pulp to 1 part of bleached sulfite pulp, together with sufficient calcium carbonate filler to give a sheet of over 30% filler content.

A coated paper which has This sheet weighed 56 pounds per ream 25x38- 500. To each side of the sheet was applied 7 pounds dry weight of an aqueous mixture comprising fine coating clay 100 parts, starch gum adhesive 16 parts, and 5 parts of 138 M. P. parafiln fineLv dispersed in water by means of a small quantity of mixed sorbitan mono-oleate and a sorbitan mono-oleate polyoxalkylene derivative. (The last two named compounds are well-known and eflicient dispersing agents.) The so-coated paper was dried and smoothed. It was then recoated on each side with 5 pounds dry weight of an aqueous coating composition comprising fine coatingclay 40 parts, fine coating grade calcium carbonate 60 parts, and alkali-dissolved casein 12 parts. The paper was then dried and supercalendered in a conventional manner. The finished product was of very excellent quality in all respects, including especially uniformity of surface characteristics and fastness of the coating to rubbing.

I claim:

1. In the process of preparing an aluminum coated paper web in which a layer of wax-free aqueous coating composition comprising finely divided aluminum and a water-dispersible adhesive is applied over a previously applied and dried base coating of aqueous mineral paper coating composition comprising a mineral pigment, the improvement which consists in incorporating a waxy material of low acid value, which waxy material is at best saponifiable only with difiiculty, in the mineral paper coating composition from which the base coating is to be made, the amount by weight of waxy material employed being from 1 to 15% of the weight of the total solids content of said base coating.

2. In the process of preparing a metal coated paper web in which a layer of wax-free aqueous coating composition comprising finely divided metal and a water-dispersible adhesive is applied over a previously applied and dried base coating of aqueous mineral paper coating composition comprising a mineral pigment, the improvement which consists in incorporating a substantially neutral waxy material in the mineral paper coating composition from which the base coating is to be made, the amount by weight of waxy material employed being from 1 to 15% of the weight of the total solids content of said base coating.

3. In the process of double-coating paper webs in which a layer of wax-free aqueous coating composition comprising finely divided pigmentary mineral matter and water-dispersible adhesive is applied over a previously applied and dried base coating of mineral paper coating composition comprising a mineral pigment, the improvement which consists in incorporating a waxy material of low acid value, which waxy material is at best saponifiable only with difficulty, in the mineral paper coating composition from which the base coating is to be made, the amount by weight of waxy material employed being from 1 to 15% of the weight of the total solids content of said base coating.

4. The improved process defined in claim 1, in which the waxy material is parafiln wax.

5. The improved process defined in claim 1, in which the waxy material is spermaceti.

6. The improved process defined in claim 1, in which the waxy material is a wax-like hydrogenated castor oil product.

7. The improved process defined in claim 1, in which the waxy material is dispersed in the aqueous mineral paper coating composition with the aid of a dispersing agent.

8. Process of preparing aluminum coated paper, which comprises applying to a surface of a paper base a layer of an aqueous mineral coating composition whose solids content comprises a mineral pigment, a water-dispersible adhesive and a substantially neutral waxy material, said waxy material being present in said composition in an amount by weight corresponding to from 1 to of the weight of the total solids content and being thoroughly dispersed in the aqueous medium, drying the mineral coating layer, applying over said dried mineral coating layer a layer of wax-free aqueous coating composition the solids content of which comprises finely divided aluminum, a water-dispersible adhesive and an agent effective for rendering said adhesive water-insoluble during the further processing of the product, and drying the resulting product.

9. Process of preparing a coated printing paper, which comprises applying to a surface of a paper base a layer of an aqueous mineral coating composition whose solids content comprises a min-- eral pigment, a water-dispersible adhesive and a substantially unsaponifiable waxy material, said waxy material amounting to from about 4 to about 10% by weight of the total solids content and being thoroughly dispersed in the aqueous medium, drying the mineral coating layer, applying over said dried mineral coating layer a layer of wax-free aqueous coating composition the solids content of which comprises finely divided paper coating pigment, a water-dispersible adhesive and an agent effective for rendering said adhesive water-insoluble during the further processing of the product, and drying the resulting product.

10. A coated paper product comprising a paper base, on at least one side of said paper base a layer of coating comprising adhesive, mineral pigment, and from 1 to 15% by weight of substantially neutral waxy material, said Waxy material being homogeneously dispersed through said layer, and over said layer a top coating comprising the dried residue of a wax-free aqueous dispersion of an adhesive substance having an initial dispersibility in water and finely divided paper coating pigment.

11. Product defined in claim 10, in which the top coating contains finely divided aluminum.

12. Product defined in claim 10, in which the top coating contains aluminum flake and is water-insoluble.

13. Product defined in claim 10, in which the top coating is water-insoluble.

14. Product defined in claim 10, in which the waxy material is a mineral wax.

15. Product defined in claim 10, in which the waxy material is a petroleum wax.

FREDERICK H. FROST.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,614,611 Grimm Jan. 18, 1927 2,069,786 Meulen Feb. 9, 1937 2,083,441 Frost June 8, 1937 2,106,709 Baker Feb. 1, 1938 2,117,256- Rafton May 10, 1938 2,192,488 Reilly Mar. 5, 1940 2,241,174 Bachelder May 6, 1941 2,362,884 Clark Nov. 14, 1944 2,442,972 Edelstein June 8, 1948 

8. PROCESS OF PREPARING ALUMINUM COATED PAPER, WHICH COMPRISES APPLYING TO A SURFACE OF A PAPER BASE A LAYER OF AN AQUEOUS MINERAL COATING COMPOSITION WHOSE SOLIDS CONTENT COMPRISES A MINERAL PIGMENT, A WATER-DISPERSIBLE ADHESIVE AND A SUBSTANTIALLY NEUTRAL WAXY MATERIAL, SAID WAXY MATERIAL BEING PRESENT IN SAID COMPOSITION IN AN AMOUNT BY WEIGHT CORRESPONDING TO FROM 1 TO 15% OF THE WEIGHT OF THE TOTAL SOLIDS CONTENT AND BEING THOROUGHLY DISPERSED IN THE AQUEOUS MEDIUM, DRYING THE MINERAL COATING LAYER, APPLYING OVER SAID DRIED MINERAL COATING LAYER A LAYER OF WAX-FREE AQUEOUS COATING COMPOSITION THE SOLIDS CONTENT OF WHICH COMPRISES FINELY DIVIDED ALUMINUM, A WATER-DISPERSIBLE ADHESIVE AND AN AGENT EFFECTIVE FOR RENDERING SAID ADHESIVE WATER-INSOLUBLE DURING THE FURTHER PROCESSING OF THE PRODUCT, AND DRYING THE RESULTING PRODUCT. 